This invention relates to bladed rotors for use in fluid flow machines and, more particularly, to fabrication of a composite blade root for retention within the periphery of a rotatable hub.
The invention herein described was made in the course of or under a contract, or a subcontract thereunder, with the United States Department of the Air Force.
Significant advances have been made in replacing the relatively heavy, homogeneous metallic blades of turbomachinery with lighter composite materials. The primary effort in this regard has been toward the adoption of high strength (high modulus of elasticity) filaments composited in a lightweight matrix. Early work involved glass fibers, while recent efforts have utilized boron, graphite and other synthetic filaments which are capable of providing the necessary stiffness to the turbomachinery blades.
Many problems have confronted the efforts to utilize these filaments but, to a large extent, they have been overcome. However, at least one difficult problem remains to be solved, that being the design of a suitable connection capable of transmitting the blade gas, centrifugal and impact loads to a rotatable hub or disc. Dovetail attachments presently represent the most expeditious and reliable method of affixing the blades to the hub. For composite blades, this creates a difficulty in that composite filament structures are least effective at fiber transitions or edges.
Typically, a blade is formed of a plurality of sheets or laminates of collimated filaments embedded in a lightweight matrix, the sheets being appropriately contoured and laminated so as to form the desired airfoil structural shape. To form a dovetail at the blade root, several approaches have been taken. One approach is to splay the individual filament laminates to shape the dovetail and fill the voids therebetween with wedges of filler material to provide a dense, load-carrying capability. The problem is that, as mentioned previously, composite laminates abhor transitions or discontinuities and the resulting structure may tend to be weaker than desired at the transition into the dovetail region.
A second approach has been to bring the composite filament laminates down from the airfoil section tip, wrap them essentially 180.degree. around a pin, and then route them back into the airfoil section. The wrapped pin is then inserted into a cylindrical aperture formed on the hub rim to retain the blade. The problem in this approach is that the laminates on the outside of the blade are forced to carry the majority of the blade loading, while the inner laminates contribute little, if anything, to the blade loadcarrying potential. However, the pin root concept offers an advantage over the splayed root in that the cylindrically shaped blade root is capable of rotation within its associated slot if the slot is appropriately relieved. This is an important characteristic when considering the foreign object impact tolerance of a blade, since it is much preferable to have a blade which will swing under lateral impact loads than one which is rigidly fixed and must be fabricated to withstand large lateral bending moments without filament fracture or delamination. Thus, a cylindrical root is preferable to any other shape in this regard.